The present invention relates in general to a load converter, and more particularly to a load converter, wherein a force-sensitive and deformable section thereof consists of structure composed of fixed portions, movable portions and a pair of thin, annular parallel-spaced, flexible portions extending so as to connect the fixed and movable portions together.
A load converter of the type described has been used to determine, for example, the level of force applied from a pulverulent body in a silo to an inner surface thereof, and the level of force applied to an outer surface of a shield excavator while it is in operation. Machines utilizing such a load converter in combination with an automatic controller have been manufactured increasingly. The load converter used for such purposes is required to have, in addition to the liner characteristics and non-hysteresis characteristics which are essential to a regular load converter, the rigidity of a detector portion thereof, less sensitivity to torque and to the other forces, such as a frictional force, than the force in the direction of the pressure to the detector section, and substantially the same level of outputs irrespective of the positions of the application points on the detector section.
A load converter which meets these requirements is disclosed in Japanese Patent Publication No. 57-7657, published Feb. 12, 1982. The construction and technical problems of this known load converter will be described first with reference to FIG. 5 prepared on the basis of the disclosures in the above patent publication.
In FIG. 5 which shows the conventional load converter, a detector section has a pressure receiving member 5, a pair of disc type members 6, 7 and a seat 10, and is mounted on the inner side of a case 1. The two disc type members 6, 7 are hollowed out from one surface of each thereof to form recesses 6d, 7d, which define thin, annular, flexible portions 6c, 7c supported at their respective both ends on rigid parts 6a, 6b, 7a and 7b. These two disc type members 6, 7 are abutted on each other in such a manner that the recesses 6d, 7d communicate with each other, to thereby form structure having flat, parallel plates, which consist of the two thin annular, flexible portions 6c, 7c. The disc type members 6, 7 are joined to the pressure receiving member 5 with bolts 8, and to the seat 10 with bolts 9. The seat 10 is joined to the case 1 with bolts 3. A clearance between the case 1 and pressure receiving member 5 is sealed with an elastic body 4 having a high elasticity.
If the pressure receiving member 5 receives a load on its upper surface, the pressure receiving member 5 and the associated inner rigid parts 6a, 7a joined to the pressure receiving member 5 are pressed downward, and the outer rigid parts 6b, 7b are joined to the seat 10 supported on the case such a manner that the flexible portions 6c, 7c are displaced in the vertical direction alone substantially without inclining the movable portion. If strain gauges 11a-11d are pasted on outer surfaces of inner and outer joint regions of the flexible thin portion 7c, the strain gauges 11b, 11c sense expansion strain, and the strain gauges 11a, 11d contraction strain. Therefore, when a Wheatstone bridge is formed by these four strain gauges 11a-11d, strain ascribed to the deformation of the flexible thin portions 6c, 7c which causes the movable portion to be inclined, is not detected. Accordingly, only such strain that is caused by the deformation of the movable portion due to the vertical movements thereof can be detected irrespective of positions on which a load is applied. Referring to the drawing, reference numeral 12 denotes a terminal board, and reference numeral 13 denotes a lead wire.
However, in the known structure shown in FIG. 5, even a very small error in the positions of the strain gauges has a great influence upon the results of measurement. Therefore, it is very difficult to carry out a strain gauge pasting operation accurately. The use of a load converter is as referred to previously. When a load converter is used in a silo, it often receives external force far greater than that estimated in advance, at such time that a bridge of a pulverulent body therein is destroyed suddenly altogether. When a load converter is used in a shield excavator, it often receives such external force at such time that the excavator encounters a sudden ejection of high-pressure water in the portion of the soil being dug which is in a boundary between two sections of different geological features. When a load converter, which has flat and parallel plates with joint regions at both end portions of the plates are bent at right angles as shown, receives such an external force, stress concentration having a peak stress concentration factor of about 5 occurs at these right-angled joint regions. Such a stress concentration necessarily has a bad effect on the detecting performance of a load converter. In the known load converter, the most important portion thereof does not have the necessary strength to withstand such an overload, whereas the remaining portion thereof has sufficiently high strength with respect thereto. Thus, there is yet room for improvement of the known load converter.